1. Field of the Invention
The present invention relates to an optical interference apparatus which obtains, in a non-contacting manner, information regarding an object to be examined by making use of optical interference.
2. Description of the Related Art
Conventionally, optical interference phenomena have been widely utilized for measuring a very small change in distance between objects, the surface shape of an object, or the like. Of various apparatuses utilizing optical interference phenomena, measurement apparatus utilizing a Michelson interferometer, which can readily produce an optical interference phenomenon, are widely known. The Michelson interferometer includes a half mirror or beam splitter which optically splits a light beam into two light beams propagating in two directions. A light beam emitted from a light source is optically split into two light beams by means of the beam splitter disposed on the optical path of the emitted light beam. One of the two light beams reaches an object to be examined, is reflected by the object, and reaches the beam splitter as measurement light. The other light beam is reflected by a reference mirror, and reaches the beam splitter as reference light. Thus, interference light is produced as a result of optical interference between the measurement light and the reference light. Since this interference light changes greatly depending on the state of scatter reflections of the measurement light at the object, the above-described measurement can be performed through observation of the interference light.
As described above, in the Michelson interferometer, various measurements are performed on the basis of generated interference light. Therefore, efficient use of light emitted from the light source, as the measurement light and the reference light, is important. In other words, if the light emitted from the light source cannot be utilized efficiently, measurement accuracy decreases. In general, the beam splitter used in the Michelson interferometer is designed such that the ratio between the quantity of transmitted light and the quantity of reflected light becomes 1:1. Thus, 50% of the light emitted from the light source passes through the beam splitter and reaches the object, and 50% of the emitted light is reflected by the beam splitter and reaches the reflection mirror.
Further, 50% of the measurement light from the object passes through the beam splitter and propagates toward the light source, and 50% of the measurement light is reflected by the beam splitter toward a light detector. Meanwhile, 50% of the reference light from the reference mirror passes through the beam splitter and propagates toward the light detector, and 50% of the reference light is reflected by the beam splitter toward the light source. Accordingly, the respective quantities of the measurement light and reference light actually reaching the light detector are only 25% of the quantity of the light emitted from the light source. Therefore, when the conventional Michelson interferometer configured as described above is used for measurement of an object to be examined, the light emitted from the light source cannot be utilized effectively.
In view of the above, recently, there has been used a Michelson interferometer in which, in order to increase the use efficiency of light, a polarizing beam splitter is used as the beam splitter, and a ¼ λ plate is provided between the beam splitter and an object to be examined and between the beam splitter and the reference mirror. In this improved Michelson interferometer, when a light beam emitted from the light source reaches the polarizing beam splitter, the light is split into two polarized light beams whose polarization planes perpendicularly intersect each other. One polarized light beam passes through the beam splitter, and the other polarized light beam is reflected by the beam splitter. The light beam having passed through the polarizing beam splitter passes through the corresponding ¼ λ plate, and reaches the object. Measurement light from the object passes the ¼ λ plate and reaches the polarizing beam splitter, at which the measurement light has a 90°-rotated polarization plane. Meanwhile, the light beam reflected by the beam splitter passes through the corresponding ¼ λ plate, and reaches the reference mirror. Reference light from the reference mirror passes the ¼ λ plate and reaches the polarizing beam splitter, at which the reference light has a 90°-rotated polarization plane.
Since each of the ¼ λ plates rotates the polarization plane of the corresponding light beam by 90°, the polarizing beam splitter can reflect all (100%) the measurement light, and can pass all (100%) the reference light therethrough. Thus, the respective quantities of the measurement light and reference light reaching the light detector become equal to 50% of the quantity of the light emitted from the light source. Therefore, in the improved Michelson interferometer, the light emitted from the light source can be utilized more effectively. However, even in this case, since only 50% of the light emitted from the light source is used, desire has arisen to use the light more efficiently and improve the measurement accuracy.
Incidentally, in the medical field, use of optical coherence tomography has recently attracted attention, as it facilitates non-invasive measurement of the interior of a living organism. In optical coherence tomography, use of near infrared interferable light attains micron-order imaging of very small regions. Optical coherence tomography has been put into practice particularly in the fields of intracatheters and endoscopes, and Japanese Patent Application Laid-Open (kokai) No. 2001-125009 discloses an endoscope which makes use of a Michelson interferometer. This endoscope enables a physician to view the surfaces of the body cavity wall of a patient by use of visible light or excitation light and to observe the interior of an affected part on the basis of a tomogram obtained by optical coherence tomography using near infrared interferable light, to thereby perform thorough examination. Therefore, cancer, tumor, or other pathological conditions can be detected at an early stage, accurate diagnosis can be made quickly, and stress experienced by patients can be mitigated. As described above, optical coherence tomography achieves accurate and quick diagnosis and reduces stress imposed on patients. Therefore, in recent years, studies for application of this technique to eye diseases have been actively carried out.
However, although the endoscope disclosed in the above-mentioned publication enables a physician to obtain a tomogram of an affected part, the information the physician can obtain is limited to only that regarding the profile obtained from the tomogram. Therefore, in diagnosis of a patient in terms of pathological condition and development, the physician must rely on his experience and knowledge, thereby increasing the burden imposed on the physician. In diagnosis of eye diseases, particularly an eye disease in the vicinity of the retina of the eyeball, observation of a very small area is required, thereby further increasing the burden imposed on the eye doctor. Moreover, in an eye disease involving necrosis of photoreceptor cells, such as glaucoma, accurate diagnosis may be difficult to perform on the basis of only the information regarding the profile obtained from a tomogram. Therefore, particularly in diagnosis of eye diseases, there has been keen demand for a practical measuring apparatus which makes use of optical coherence tomography and which can provide eye doctors with a greater deal of accurate information.
However, when a greater quantity of accurate information is to be provided through measurement, it is necessary to emit near-infrared interferable light beams having different wavelengths, and to detect and measure a plurality of beams of measurement light (near-infrared interferable light) from the object. In this case, for example, when the above-described improved Michelson interferometer is employed, the optical system may become complex. That is, the characteristics of an optical system composed of a polarizing beam splitter and ¼ λ plates change depending on the wavelength of incoming light (near infrared interferable light) (so called wavelength dependency). Therefore, when multi-wavelength, near-infrared interferable light is used, an optical system composed of a polarizing beam splitter and ¼ λ plates must be provided for each wavelength. In this case, the optical path from the light source to an object to be examined becomes complex, whereby adjustment for securing a proper optical path becomes extremely difficult, and thus, the adjustment work may become troublesome. In addition, because of the increased complexity of the optical system, the apparatus itself becomes larger, which is not practical.